Undefined Variable PHP/MySQL

`<?php
session_start();
include_once("scripts/connect_to_mysql.php");

if(isset($_GET['id'])){   
$id2=preg_replace('#[^0-9]#i','',$_GET['id']);
$sql = mysql_query("SELECT username FROM users where id='$id2'");
$row = mysql_fetch_array($sql);
        $username2= $row[0];
}
if(isset($_SESSION['username']))
$username=($_SESSION['username']);


$sql1 = mysql_query("SELECT * FROM users where username='$username'");
$row1 = mysql_fetch_array($sql1);
        $id1= $row1["id"];

if (isset($_POST['title'])) {
$title = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["title"]); //filtreaza         tot mai putin firstNamere si litere
$body = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["body"]); //filtreaza tot mai putin firstNamere si litere
$sql = mysql_query("INSERT INTO message (username1,username2,title,body, sentAt) 
        VALUES('$username','$username2','$title','$body',now()") or die (mysql_error());
header("location: profile.php"); 
exit();

}
?>`

error:Undefined variable: username2 , but when I print the variable in HTML i's ok .What is wrong? I'm new with this so please help me.